Acceleration sensor with protrusions facing stoppers

ABSTRACT

An integrally micromachined acceleration sensor has a mass with a surface facing a stopper. At least one protrusion projects from this surface toward the stopper. In the absence of acceleration, the protrusion is spaced apart from the stopper, but by limiting motion of the mass toward the stopper, the protrusion improves the shock resistance of the acceleration sensor. The protrusion also prevents the mass from sticking to the stopper during the fabrication process. The stopper may have a pattern of holes surrounding the protrusion, so that the protrusion is produced naturally during the wet etching process that separates the mass from the stopper. The holes also shorten the wet etching time.

This is a Divisional of U.S. application Ser. No. 11/705,763, filed Feb.14, 2007, the subject matter of which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a micromachined acceleration sensor,more particularly to an acceleration sensor with features that aid themicromachining process and improve the robustness of the sensor.

2. Description of the Related Art

Known micromachined acceleration sensors include three-axis accelerationsensors having a mass flexibly linked to a frame by beams withmicroelectronic strain detectors. Acceleration sensors of this type canbe classified into a bonded type, which is formed by micromachiningdifferent layers of the sensor on separate substrates and then bondingthe layers together, and an integral type, which is formed bymicromachining a substrate that already has a layered structure. Thepresent invention relates to a three-axis acceleration sensor of theintegral type, such as the one described in Japanese Patent ApplicationPublication (JP) No. 2004-198243.

The frame of this type of acceleration sensor includes stoppers thatlimit the motion of the mass. Because the sensor is of the integraltype, the micromachining process includes a wet etching step thatseparates the stoppers from the mass, followed by a cleaning step thatrinses the etching solution out from the space between the mass and thestoppers. The dimensions of the acceleration sensors now being producedhave become so small that after the cleaning process, the mass andstoppers may still be joined by drops of rinsing solution. This leads toa fabrication problem, because as the remaining rinsing solution dries,its surface tension draws the mass toward the stoppers and may cause themass and stoppers to stick together.

JP 2004-294401 (U.S. Patent Application Publication No. 20040187592)discloses a single-axis capacitive acceleration sensor in which thebottom surfaces of the mass and moving electrodes are etched laterallyin such a way as to leave protrusions to prevent the bottom surfacesfrom sticking to the base layer of the substrate, but the formation ofthese protrusions requires laterally convex extensions of the mass andelectrodes. Similar protrusions between the mass and stoppers of athree-axis acceleration sensor could be considered, but in a three-axissensor the necessary laterally convex extensions would undesirably limitthe freedom of motion of the mass. If the lateral dimensions of the masswere to be reduced to regain the necessary freedom of motion, theresulting loss of inertial mass would reduce the sensitivity of thesensor, which would also be undesirable.

SUMMARY OF THE INVENTION

An object of the present invention is to prevent the mass of anacceleration sensor from sticking to the stoppers during the fabricationprocess.

Another object of the invention is to shorten the fabrication process.

Still another object is to increase the robustness of the accelerationsensor.

Yet another object is to increase the sensitivity of the accelerationsensor.

The invented acceleration sensor has a patterned layer including a massattachment section, a peripheral attachment section, at least one beamflexibly linking the mass attachment section to the peripheralattachment section, and at least one stopper contiguously joined to theperipheral attachment section. A mass having a surface facing thestopper is joined to the mass attachment section by a first joininglayer. A frame surrounding the mass is joined to the peripheralattachment section by a second joining layer.

The surface of the mass that faces the stopper has at least oneprotrusion that protrudes toward the stopper. Absent acceleration, theprotrusion is spaced apart from the stopper. Preferably, there are aplurality of such protrusions, which may be arranged in atwo-dimensional array extending over substantially the entire surface ofthe mass that faces the stopper. The protrusions are preferably made ofthe same material as the first and second joining layers.

The stopper preferably has a plurality of holes positioned such thateach protrusion is disposed between geometric projections of at leasttwo of the holes onto the surface of the mass.

The invented acceleration sensor may be fabricated by a method includingthe steps of:

preparing a substrate having a first layer, a second layer, and ajoining layer through which the first layer is joined to the secondlayer;

patterning the first layer to form a mass attachment section, aperipheral attachment section surrounding and spaced apart from the massattachment section, at least one beam flexibly linking the massattachment section to the peripheral attachment section, and at leastone stopper contiguously joined to the peripheral attachment section andspaced apart from the mass attachment section and the beam;

patterning the second layer to form a mass spaced apart from thestopper, having a surface facing the stopper, and a frame surroundingand spaced apart from the mass; and

selectively removing the joining layer to leave a first joining layerjoining the mass to the mass attachment section, a second joining layerjoining the frame to the peripheral attachment section, and at least oneprotrusion protruding from said surface of the mass toward the stopper,the protrusion being spaced away from the stopper.

The step of selectively removing the joining layer is preferably carriedout by wet etching.

The step of patterning the first layer preferably also forms a pluralityof holes facing respective areas on said surface of the mass, eachprotrusion being disposed between at least two of these areas.

The protrusions prevent the mass from sticking to the stopper during thefabrication process.

The holes formed in the stopper shorten the fabrication process byfacilitating the etching of joining-layer material between the mass andstopper and naturally leading to the formation of the protrusions.

The protrusions increase the robustness of the acceleration sensor byshortening the distance through which the mass can travel toward thestopper, thereby reducing the risk of beam or stopper damage caused byshock.

By slightly increasing the amount of mass, the protrusions increase thesensitivity of the sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

In the attached drawings:

FIG. 1 is a perspective view of an acceleration sensor embodying thepresent invention;

FIG. 2 is an upper plan view of the acceleration sensor in FIG. 1;

FIG. 3 is a sectional view through line AA′ in FIG. 1;

FIG. 4 is a sectional view through line BB′ in FIG. 1;

FIG. 5 is a partial perspective view of the mass in FIG. 1;

FIG. 6 is an upper plan view of the first layer in FIG. 1;

FIG. 7 is an upper plan view of the joining layer in FIG. 1;

FIG. 8 is an upper plan view of the second layer in FIG. 1;

FIGS. 9, 10, 11, 12, and 13 are sectional views illustrating steps inthe fabrication of the acceleration sensor in FIG. 1;

FIGS. 14, 15, 16, and 17 are plan views illustrating possible layouts ofthe holes and protrusions in FIG. 1;

FIG. 18 is a perspective view of a conventional acceleration sensor;

FIG. 19 is an upper plan view of the conventional acceleration sensor;

FIG. 20 is a sectional view illustrating a starting state in thefabrication of the conventional acceleration sensor;

FIGS. 21A, 22A, and 23A are sectional views through line AA′ in FIG. 18,illustrating successive steps in the conventional fabrication process;

FIGS. 21B, 22B, and 23B are corresponding sectional views through lineBB′ in FIG. 18; and

FIGS. 21C, 22C, and 23C are corresponding upper plan views of variouslayers in FIG. 18.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the invention will now be described with reference to theattached drawings, in which like elements are indicated by likereference characters.

A three-axis acceleration sensor embodying the present invention isshown in perspective view in FIG. 1. The acceleration sensor isfabricated in a substantially square substrate having a first layer orpatterned layer 101 joined by a joining layer 102 to a second layer 103.The peripheral section 110 of the acceleration sensor includes aperipheral attachment section 111 formed in the first layer 101, joinedthrough the joining layer 102 to a frame 113 formed in the second layer103. Four beams 120 extend in the first layer 101 from the peripheralattachment section 111 toward the central section 130 of theacceleration sensor. The central section 130 includes a mass attachmentsection 131 formed in the first layer 101, joined through the joininglayer 102 to a mass 133 formed in the second layer 103. Each beam 120 isintegrally attached at a first end 121 to the peripheral attachmentsection 111 and a second end 122 to the mass attachment section 131, andincludes piezoresistive elements (not shown) for sensing strain when thebeam 120 bends.

The part of the joining layer 102 that joins the mass attachment section131 to the mass 133 will be referred to as the first joining layer 132;the part of the joining layer 102 that joins the peripheral attachmentsection 111 to the frame 113 will be referred to as the second joininglayer 112.

Four stoppers 140 are disposed in the first layer 101 at the four innercorners of the peripheral attachment section 111, to which they areconnected. Each stopper 140 has the shape of a right isosceles triangle.A plurality of holes 141 are formed in each stopper 140, extending fromits top surface to its bottom surface.

The mass 133 has for square lobes, each with a surface that extendspartly beneath one of the stoppers 140. A plurality of protrusions 150extend from this surface toward the facing undersurface of the stopper140. As shown by the top plan view in FIG. 2, the protrusions 150project toward points disposed between the holes 141 in the stopper 140.

The mass attachment section 131 is spaced apart from the sides of thebeams 120, and from the stoppers 140. The four lobes of the mass 133 arespaced apart from the frame 113, and absent acceleration, theprotrusions 150 are spaced apart from the stoppers 140, as shown in FIG.3. The central part of the mass 133 is widely spaced apart from theframe 113 by cavities below the beams 120, as shown in FIG. 4.

The protrusions 150 have a square pyramidal shape, as best seen in FIG.5. This drawing shows part of one lobe of the mass 133. The facingstopper 140 is omitted from FIG. 5 for clarity, but the part of thesurface of the mass 133 that faces the stopper 140 is bounded by thedotted line 151. Circular dotted lines in FIG. 5 define areas 152 facingthe holes 141 in the stopper 140. The protrusions 150 are disposedbetween these areas 152, which are geometric projections of the holes,and the protrusions 150 are oriented so that their sides face towardthese areas 152.

The greater the height of the protrusions 150, the less the mass 133 canmove toward the stoppers 140. The height of the protrusions 150 shouldbe chosen to allow enough motion for acceleration to be sensed but notso much motion that the beams 120 might break under strong acceleration.

Most of the part of the square lobe of the mass 133 that does not facethe stopper 140 is joined by the first joining layer 132 to the massattachment section 131, as shown at the back of FIG. 5. The spacebetween the dotted line in FIG. 5 and the first joining layer 132corresponds to the space between the mass attachment section 131 andstopper 140 in FIGS. 1 and 2.

Although the substrate layers 101, 102, and 103 are unitarily contiguousand cannot be separated from one another, strictly for explanatorypurposes, FIGS. 6, 7, and 8 show top plan views of the three layersseparately.

The first layer 101, shown in FIG. 6, is a silicon layer with apreferred thickness in the range from three to eight micrometers (3-8μm). The mass attachment section 131 is separated from the beams 120 andstoppers 140 by trenches 401 with a preferred width of 10-25 μm.

The joining layer 102, shown in FIG. 7, is a silicon oxide layer with apreferred thickness of 1-3 μm. The joining layer 102 includes not onlythe second joining layer 112 that joins the peripheral attachmentsection 111 to the frame 113 and the first joining layer 132 that joinsthe mass attachment section 131 to the mass 133, but also theprotrusions 150. A plurality of protrusions 150 are formed below eachstopper 140 to ensure that, if acceleration drives the mass 133 towardthe stoppers 140 at an angle such that the protrusions 150 strike thestopper 140 in only one corner of the sensor, the impact force will notbe concentrated on just one protrusion 150, which might damage thesensor.

The first joining layer 132 in FIG. 7 has the same plan geometry as themass attachment section 131 in FIG. 6, and the second joining layer 112has the same plan geometry as the peripheral attachment section 111.Below the beams 120 and stoppers 140, the joining layer 102 is removedduring the fabrication process, except for the protrusions 150.

The second layer 103, shown in FIG. 8, which includes the peripheralframe 113 and mass 133, is a silicon layer with a preferred thickness of200-400 μm. The shape of the mass 133, with large outer lobes and asmaller central part, is designed to maximize its total size and henceits total inertial mass, while also maximizing the length of the beams;both of these factors enhance the sensitivity of the accelerationsensor. The thickness of the mass 133 is preferably 8-15 μm less thanthe thickness of the frame 113. This thickness difference, best seen inFIG. 3, corresponds to the maximum distance through which the mass 133can move from its rest position in the direction away from the stoppers140.

A fabrication process for this acceleration sensor will now be describedwith reference to FIGS. 9 to 13, which correspond to sections throughline AA′ in FIG. 1.

The fabrication process starts from a silicon-on-insulator (SOI) wafersubstrate having a first layer 101, a joining layer 102, and a secondlayer 103 as shown in FIG. 9. The joining layer 102 may be a so-calledburied oxide layer. Although only one acceleration sensor is shown inthe drawings, normally many acceleration sensors are fabricatedsimultaneously in the same wafer.

First, standard microelectronic semiconductor fabrication methods areused to form piezoresistive elements (not shown) in the part of thefirst layer 101 that will become the beams 120. In addition, the firstlayer 101 is anisotropically etched to form the trenches 401 shown inFIG. 6 that define the peripheral attachment section 111, beams 120,mass attachment section 131, and stoppers 140, and to form a pluralityof holes 141 in each stopper 140. The result is illustrated in FIG. 10.

Next, the underside of the second layer 103 of the wafer is etched to adepth of 8-15 μm in the region that will become the mass 133, as shownin FIG. 11.

The underside of the second layer 103 is then further etched by ananisotropic etching process to form trenches 502 as shown in FIG. 12that separate the mass 133 from the frame 113 and that separate thelobes of the mass 133 from each other. This etching process removes allparts of the second layer 103 from beneath the beams 120 and from asquare annular ring just inside the frame 113; the etching process endsat the joining layer 102, which is not etched.

Finally, a wet etching process is performed by immersing the wafer in anetching fluid that etches the silicon oxide of the joining layer 102 butdoes not etch the silicon of the first and second layers 101 and 103(more precisely, the etching fluid etches silicon oxide much morerapidly than silicon). The etching fluid easily reaches the part of thejoining layer 102 exposed by the trenches 401 and 502 formed in thepreceding steps and removes all of the joining layer 102 from the areabeneath the beams 120 and the area between the frame 113 and mass 133.As wet etching is isotropic, the etching process also proceeds laterallyfrom these trenches 140, 152 into the spaces between the stoppers 140and mass 133. Additional etching fluid reaches this space through theholes 141 in the stoppers 140, and by etching isotropically from theends of the holes 141, excavates a cavity beneath each hole. The cavityis wider at the top (near the hole) than at the bottom (on the surfaceof the mass 133). As these cavities grow, they shape the protrusions150. If the etching conditions are properly selected, protrusions 150 ofthe desired height will be left on the surfaces of the mass 133 beneaththe stoppers 140, as shown in FIG. 13. In experiments by the inventor,appropriate protrusions 150 were formed with a total wet etching time ofabout seventy minutes.

After wet etching, the completed acceleration sensor is cleaned to rinseaway the etching fluid, and then dried. The protrusions 150 prevent themass 133 from sticking to the stoppers 140 during the drying process, sothe dried acceleration sensor can immediately be diced from the waferand mounted in an appropriate package.

The wet etching step may be performed as a single continuous process, oras a series of short etch-rinse cycles. The latter strategy promotesetching by removing the etched silicon oxide material at the end of eachcycle and replacing the spent etching fluid, which has already reactedwith the silicon oxide, with fresh etching fluid. Etching may be furtherpromoted by immersing the wafer in a surfactant solution before eachetching cycle, to reduce the surface tension of the etching fluid andrinsing fluid and enable etching to proceed efficiently even in thenarrow space between the mass 133 and stoppers 140.

As the wet etching process forms protrusions 150 not in the areas 152directly beneath the holes 141 but at locations between these areas, ifacceleration moves the mass 133 toward the stoppers 140 during operationof the acceleration sensor, the protrusions 150 will strike the surfaceof the stoppers 140, as desired, instead of entering the holes 141.

The number of holes 141 and protrusions 150 per stopper 140 is notlimited to the numbers shown in FIGS. 1, 2, 5, and 6; a larger numbermay be formed, as illustrated in FIG. 14, for example. The preferreddiameter of the holes 141 is 3-4 μm, and the preferred spacing betweenthe edges of adjacent holes 141 is 4.5-5.5 μm. The center-to-centerspacing of the holes 141 is then approximately 8.5 μm.

In the design stage, the holes 141 can be laid out by defining two holeson an imaginary reference line, then translating the line so that onehole occupies the location of the other hole, rotating the line byninety degrees to define a new hole, and repeating this process untilall the necessary holes have been defined. Alternatively, a unit cell Aof four holes 141 surrounding one protrusion 150 can be defined; thenthe unit cell can be stepped horizontally and vertically to definefurther holes 141.

The layout is not limited to the square cell A shown in FIG. 14. Atriangular cell A with three holes 141 surrounding one protrusion 150can be used, as shown in FIG. 15, or a hexagonal cell with six holes 141surrounding one protrusion 150 can be used, as shown in FIG. 16. Theresulting protrusions 150 will then have a triangular pyramidal shape ora hexagonal pyramidal shape, as shown in FIGS. 15 and 16. Increasing thenumber of holes around each protrusion 150 increases the etching speed,so to shorten the etching time, the number of holes 141 may be increasedstill further. FIG. 17 shows a unit cell A with eight holes 141, forexample, which produces protrusions 150 with an octagonal pyramidalshape.

Increasing the number of holes 141 also weakens the stoppers 140,however, and therefore reduces the ability of the sensor to withstandshock. The number of holes 141 per protrusion 150 and hence the shape ofthe protrusions 150 should be selected by balancing requirements forquick etching against requirements for a robust acceleration sensor. Thesquare pyramidal shape shown in FIGS. 5 and 14 is thought to representan appropriate compromise.

It not necessary to tile the entire surface of a stopper 140 with unitcells A as in FIGS. 14 to 17. A few unit cells may be placed at selectedlocations in the stopper 140. This provides another way to achieve anappropriate balance between robustness and short etching time.

During operation, as noted above, the protrusions 150 reduce thedistance through which the mass section 130 can travel in the directionperpendicular to the surfaces of the stoppers 140. This has thedesirable effect of reducing the risk of damage to the accelerationsensor if strong acceleration drives the mass 133 forcefully against thestoppers 140.

For comparison, FIG. 18 shows a conventional acceleration sensor of thetype described in JP 2004-198243, comprising a first layer 701, joininglayer 702, second layer 703, peripheral section 710, beams 720, masssection 730, and stoppers 740 similar to the corresponding elements inFIG. 1, except that the stoppers 740 lack holes. FIG. 19 shows a planview of the first layer 701.

The fabrication process for this conventional acceleration sensor isvirtually identical to the fabrication process for the inventiveacceleration sensor described above, except that because of the lack ofholes in the stoppers 740, the wet etching step takes longer and doesnot leave protrusions.

The conventional fabrication process begins from an SOI wafer substrateas illustrated in FIG. 20. The first layer 701 is anisotropically etchedto define the upper parts of the peripheral section 710 and mass section730, the beams 720, and the stoppers 740 as shown in FIG. 21A (asectional view through line AA′ in FIG. 18), FIG. 21B (a sectional viewthrough line BB′ in FIG. 18), and FIG. 21C (a top plan view of the firstlayer 701). Next the second layer 703 is anisotropically etched todefine the lower parts of the peripheral section 710 and mass section730, as shown in sectional views in FIGS. 22A (another view through lineAA′) and 22B (another view through line BB′) and in a bottom plan viewin FIG. 22C. Finally, a wet etching process is performed to remove thejoining layer 702 from the undersides of the beams 720 and stoppers 740,as shown in sectional views in FIGS. 23A (again through line AA′) and23B (again through line BB′) and a plan view of the resulting patternedjoining layer 702 in FIG. 23C.

The total wet etching time in the conventional fabrication process, whenperformed under the same wet etching conditions as in the aboveembodiment, is about eighty minutes. The present invention thus reducesthe wet etching time by about ten to thirteen percent. Moreover, whenthe conventional acceleration sensor is dried after wet etching andcleaning, the mass 730 sometimes sticks to the stoppers 740, as notedabove, and further time is required to deal with this problem. Theinvention thus leads to a quicker manufacturing process, as well as amore robust and more sensitive sensor.

The foregoing represents one preferred embodiment of the invention.Those skilled in the art will recognize that many other embodiments andvariations are possible within the scope of the invention, which isdefined in the appended claims.

1. A method for fabricating an acceleration sensor, comprising:preparing a substrate having a first layer, a second layer, and ajoining layer through which the first layer is joined to the secondlayer; patterning the first layer to form a mass attachment section, aperipheral attachment section surrounding and spaced apart from the massattachment section, at least one beam flexibly linking the massattachment section to the peripheral attachment section, and at leastone stopper contiguously joined to the peripheral attachment section andspaced apart from the mass attachment section and the beam; patterningthe second layer to form a mass spaced apart from the stopper and havinga surface facing the stopper, and a frame surrounding and spaced apartfrom the mass; and selectively removing a part of the joining layer toleave a first joining layer joining the mass to the mass attachmentsection, a second joining layer joining the frame to the peripheralattachment section, and at least one protrusion protruding from thesurface of the mass toward the stopper, the protrusion being spacedapart from the stopper.
 2. The method of claim 1, wherein theselectively removing a part of the joining layer is carried out by wetetching.
 3. The method of claim 1, wherein the patterning the firstlayer further includes forming a plurality of holes facing respectiveareas on the surface of the mass, and the at least one protrusion isformed between at least two of the areas in the step of selectivelyremoving a part of the joining layer.
 4. The method of claim 1, whereinthe at least one protrusion is formed so as to have a square pyramidalshape, a triangular pyramidal shape, a hexagonal pyramidal shape, or anoctagonal pyramidal shape.